Rainwater harvesting (RWH) has been practiced in Ethiopia since ancient times, but it has shown little development, because of inefficient techniques. Most efforts to capture rainwater did not show significant results owing to poor design and implementation resulted from slow technical development. This paper details design improvements tested on a demonstration site as well as an analysis of operational parameters. Similar, rainwater quality improvement techniques applied for the system are also discussed. Various scenarios were studied relating design and operating parameters for replicability and decision-making before construction stage. Common components of existing RWH systems in Ethiopia are discussed and contrasted with the implemented demonstration RWH system. Moreover, particle separation theory and techniques are introduced as quality improvement techniques. Results of the analysis also suggest a possibility of satisfying building demand by utilizing the installed system and it also shows the potential of RWH in Ethiopia as an alternative water source.

INTRODUCTION

High population growth, rapid urbanization, climate change and competitive demand requirements are the main causes of water shortage in Ethiopia (CSA & ICFI 2012). Because of the fast rate of urbanization, urban areas of the country are currently experiencing water shortage. In the capital city, Addis Ababa, water demand exceeds the supply by about 50% forcing half of the population to have less than 12 hours access and a quarter of the population not having any daily access whatsoever (Hameeteman 2013). Research and development toward alternative water sources to provide for the demand is imperative, since dependence on conventional sources is not able to address these current challenges.

Rainwater harvesting (RWH) in Ethiopia dates back to the pre-Axumit period (560BC) (Ketsela 2009). Archeological remains of a RWH installation in an old castle of Fasil, built in the 15th and 16th centuries, provide evidence for its ancient use in Ethiopia (Getachew 1999). RWH has received renewed attention from policymakers in the past few decades as a response to frequent droughts. However, its development in the country overlooked the potential of urban application and focused only on rural areas (Getachew 1999; Awulachew et al. 2005; Ketsela 2009). Furthermore, reports on existing RWH attempts indicate that the majority of implemented systems had seepage and water quality problems, mainly due to the lack of effective system designs (Awulachew et al. 2005; Ketsela 2009). Because there was no nationally standardized engineering design of RWH system in the country until this time, prediction of operational parameters (OP) including no-water days (NWD), rainwater use efficiency (RUE), and cycle number (CN) for any given system has not been studied.

Considering the current situation in Ethiopia, this paper discusses detailed engineering design of a RWH system installed on a three-story building of Adama Science and Technology University (ASTU), for the purpose of this study. In addition, evaluation of NWD, RUE, CN, and applied water quality improvement techniques are discussed for the possibility of system replication.

STUDY AREA

The demonstration project is installed in the Chemical Engineering Building (CHEB) of Adama Science and Technology University. It is located in the city of Adama (8°33′35″N–8°36′46″ N latitude by 39°11′57″E–39°21′15″ E longitude), 100 km southeast of Addis Ababa (Figure 1).
Figure 1

Location of the Adama (BBC 2014).

Figure 1

Location of the Adama (BBC 2014).

SYSTEM COMPONENTS

Design and installation of the RWH demonstration unit took place in June 2014. It was designed considering improvements on previous existing systems in Ethiopia, which only consisted of collection and storage. On the contrary the demonstration system consisted of four basic units: collection, piping, storage, and treatment (Figure 2).
Figure 2

Schematics of the RWH system on CHEB: (a) plan view, (b) side view.

Figure 2

Schematics of the RWH system on CHEB: (a) plan view, (b) side view.

Collection area

Half of the building roof with an area of 565 m2 was used as the collection surface. Because the roof was relatively new and clean, the maximum runoff coefficient of galvanized sheet roof was 0.9 (UNEP & CEHI 2009), was used. This makes the estimated collection area to be 506 m2.

Piping network

Polyvinyl chloride (PVC) pipes were used for the main piping systems. A 0.11 m and 0.05 m diameter pipe was used as gutter or downpipes and calm inlet, respectively. A 0.11 m diameter tee was used to connect the top of the first flush tank to the pipe coming from the collection gutters (Figure 3). A 0.11 to 0.05 m diameter reducer was used at the connection point between the down pipes and lines to the storage tank. The supply and drainage were 0.025 m diameter green polyethylene pipes. A 0.05 m diameter PVC overflow pipe was installed at the same height at which the inlet pipe was connected. This setup helps to maintain the level of collected rainwater at a constant maximum level, preventing backflow and the possibility of flooding during heavy rainfall.
Figure 3

Detailed schematic of the storage tank.

Figure 3

Detailed schematic of the storage tank.

Treatment units

The system is designed to permit the rainwater to pass through the first flush tank, followed by the calm inlet, and lastly the long storage tanks used also as settling units. These units are discussed in detail in the water quality improvement section of this paper.

Storage

Two horizontal storage tanks with individual capacities of 4 m3 were installed and set to split equal amount of rainwater collected from the collection area. Horizontal type tanks were chosen to provide a more effective sedimentation area (Figure 3).

SYSTEM COST

The basic investment cost components of the system are storage tanks, piping, and labor costs. Because an existing building roof was employed as the collection roof, the largest investment cost of the RWH system was incurred by the tank cost. Results from other projects undertaken by the authors of this paper show that depending on the tank material and size, the cost of the storage tank can range from 50 to 85% of the overall system cost for systems on already existing roofs. For this case study, the tank cost accounted for 61% of the initial investment cost (Table 1). The remaining costs are summarized in Table 1.

Table 1

Cost summary of the RWH system installed at CHEB of ASTU

Cost itemCost (USD)Cost percentage share (%)
Two 4 m3 storage tanks 1,474.68 61.0 
Piping (down pipe & supply pipes) 507.47 21.0 
Labor and other accessories 436.94 18.1 
Total cost 2,419.09 100.0 
Cost itemCost (USD)Cost percentage share (%)
Two 4 m3 storage tanks 1,474.68 61.0 
Piping (down pipe & supply pipes) 507.47 21.0 
Labor and other accessories 436.94 18.1 
Total cost 2,419.09 100.0 

ENGINEERING DESIGN PARAMETERS

The RWH system design was tested under the combined effect of four major design parameters (DPs), namely collection roof area, storage tank design capacity, average daily water demand and annual average rainfall intensity. For analysis purposes, 30 years of average daily rainfall data of Adama were used (Figure 4).
Figure 4

Daily average rainfall of Adama city (EMA 2014).

Figure 4

Daily average rainfall of Adama city (EMA 2014).

Using the four DPs, the OPs were evaluated under different DP conditions. During the design period, it was discovered that the building receives its water supply from a groundwater source 2 days per week at approximately 1.5 m3/day, which is for only 114 days per annum. The collected rainwater is used for toilet flushing in the building to compensate for the demand gap. For this, the toilet water demand was estimated from the minimum and maximum number of people requiring the service per day and the rate of consumption for a single flush. The toilets installed in CHEB are conventional flushing toilets which utilize 20–40 liters/user/day (Reed & Reed 2013). The building accommodates a minimum of 20 and a maximum of 230 students and staff members in each working day. Based on these data, the RWH demonstration project was designed to provide an average daily demand of 1 m3, with 50% of the roof area having a maximum expected 100 NWDs.

OPERATING PARAMETERS

The 30 years of average daily rainwater data of Adama were provided by the Ethiopian Meteorology Agency. The performance of the system was analyzed based on the daily water balance concept with OPs of RUE, NWD, and CN. The water balance equation for Figure 3 is expressed as follows: 
formula
1
where Vt is the stored volume of rainwater (RW) in the tank (m3) at day ‘t’, Qi,t is daily roof collected RW to the tank (m3/day), Qs,t is the daily RW supply of the system (m3/day), and Qo,t is the overflow from the tank (m3/day) and ‘t’ is cumulative collection time in (days).
The daily roof RW harvested per day (Qi,t), is a function of daily rainfall depth (R, in m/day), area of collection (A, in m3) and losses due to evaporation, leakage and gutter overflow, which is represented by the dimensionless runoff coefficient of the roof (φ) (Rashidi Mehrabadi et al. 2013) and is given as: 
formula
2
The RUE is defined by the ratio of total RW supplied by the system (Qs,i) to the total daily collected RW (Qi,t) as given in Equation (3). 
formula
3
 
formula
4
Similarly, the CN, which is a parameter for measuring the amount of RW utilization per unit tank volume, is given by the ratio of total rainwater used to the tank volume (m3) given in Equation (4) (Mun & Han 2012).
The OPs are analyzed for a design requirement of satisfying a 1 m3 average daily demand with two 4 m3 storage tanks and a collection roof of 50% utilization. Furthermore, varying the collection roof area and the total collection capacity of the tanks alternative optimization scenarios were analyzed. For the combined 8 m3 system with a 50% roof area utilization, the NWD rapidly increases when the demand increases from 0.5 to 2 m3/day (Figure 5(a)). For optimal operation, the maximum expected NWDs in CHEB should be less than 114 days, which is the number of days per year where the building has access to the water line from the existing ground water system in the building.
Figure 5

NWD vs. daily supply with respect to collection roof size and storage tank capacity variation.

Figure 5

NWD vs. daily supply with respect to collection roof size and storage tank capacity variation.

With this assumption, a 1 m3 supply results in almost 94 days, which is less than the maximum expected NWD in the CHEB (Figure 5(a)). Doubling the collection roof area reduces the NWD by half to 42 days, or allows for an increase of daily consumption by 50% with only 39% increase in total system cost (Figure 5(a)). Tripling the size of storage tank with a 50% roof area utilization has the comparable result of halfing the NWD (Figure 5(b)). However, tripling the total storage tank capacity increases system costs by approximately 122%. Considering the rainfall distribution of Adama, the cost of installation and the already existing roof of CHEB, the analysis suggests that utilization of unused roof spaces is preferable to installing additional storage tanks for achieving comparable NWD savings.

Similarly, the RUE for the design system was analyzed following an increase in collection roof area and storage tank capacity. The result shows that tripling the storage volume capacity yields less than a 5% increase in RUE (Figure 6(a)). However, for the 8 m3 total storage capacity, increasing the supply to cover a demand of up to 3 m3 per day increases the RUE by up to 94%. Increasing storage capacity while keeping the roof area to 50% utilization did not significantly affect the RUE. However, doubling the roof collection area allowed the possibility of doubling the supply for an equivalent RUE, with an increase of only 39% in system costs which is much less than a 122% cost increase by increasing the storage capacity.
Figure 6

RUE vs. daily supply variation for different storage tank sizes.

Figure 6

RUE vs. daily supply variation for different storage tank sizes.

The system setup has an annual refill frequency of up to 34 times (Figure 7(a)). The CN analysis for system scaled up to 3 m3/day supply shows that doubling the collection roof area has an advantage of increasing the CN from 52 to 78 times, which increases the ability of storing significantly more rainwater per annum. It is also deduced at this point that increasing the roof collection area is more advantageous for larger supply needs.
Figure 7

CN vs. daily supply for different collection roof sizes.

Figure 7

CN vs. daily supply for different collection roof sizes.

The combined effect of varying utilized roof collection area with that of storage tank capacity is summarized for six different scenarios with three alternate supply amounts (Table 2). The results show that the minimum NWD of 27 days can be achieved by a combined effect of 100% roof utilization and 16 or 24 m3 storage tank capacity. This shows that the effect of increasing storage tank capacity contributes minimally to the reduction of NWD. For each scenario, increasing both the storage capacity and the utilized roof area reduces the NWD. In evaluating the separate effects of the roof area and storage tank capacity with respect to reducing NWD with minimal costs, it was found that utilizing the unused roof areas for collection was more economical than installing additional tanks.

Table 2

Scale-up scenarios versus operating parameters

ScenariosSupplyUtilized roof (%)Storage tankNWDRUECNCost (USD)
S1 1 m3 50 94 60.44 34 2,419.00 
S2 50 16 73 62.13 18 3,894.00 
S3 50 24 55 63.85 12 5,369.00 
S4 100 42 34.77 39 3,364.00 
S5 100 16 27 35.38 20 4,839.00 
S6 100 24 27 36.25 14 6,313.00 
S1 2 m3 50 250 77.91 44 2,420.00 
S2 50 16 240 79.65 23 3,894.00 
S3 50 24 233 81.33 15 5,369.00 
S4 100 130 60.94 69 3,364.00 
S5 100 16 121 60.34 34 4,838.00 
S6 100 24 114 61.26 23 6,313.00 
S1 3 m3 50 283 92.25 52 2,420.00 
S2 50 16 276 93.95 27 3,894.00 
S3 50 24 272 95.31 18 5,369.00 
S4 100 226 68.55 78 3,364.00 
S5 100 16 218 69.63 39 4,839.00 
S6 100 24 212 70.19 26 6,313.00 
ScenariosSupplyUtilized roof (%)Storage tankNWDRUECNCost (USD)
S1 1 m3 50 94 60.44 34 2,419.00 
S2 50 16 73 62.13 18 3,894.00 
S3 50 24 55 63.85 12 5,369.00 
S4 100 42 34.77 39 3,364.00 
S5 100 16 27 35.38 20 4,839.00 
S6 100 24 27 36.25 14 6,313.00 
S1 2 m3 50 250 77.91 44 2,420.00 
S2 50 16 240 79.65 23 3,894.00 
S3 50 24 233 81.33 15 5,369.00 
S4 100 130 60.94 69 3,364.00 
S5 100 16 121 60.34 34 4,838.00 
S6 100 24 114 61.26 23 6,313.00 
S1 3 m3 50 283 92.25 52 2,420.00 
S2 50 16 276 93.95 27 3,894.00 
S3 50 24 272 95.31 18 5,369.00 
S4 100 226 68.55 78 3,364.00 
S5 100 16 218 69.63 39 4,839.00 
S6 100 24 212 70.19 26 6,313.00 

WATER QUALITY IMPROVEMENT

The water quality was one of the reasons for failure on previously installed rainwater collection systems in Ethiopia. During the design and installation of the demonstration system, simple engineering solutions were implemented along the flow path from the roof to the storage tank. Three design improvements were considered to improve the quality of the collected water including a flush tank, calm inlet, and sufficient sedimentation area.

First flush tank

With the roof sloping to the center of the building, absence of trees around the roof, and sufficient building height, it was assumed that exposure to large contaminants was minimal. Therefore, the need for screen filters at the inlets of the gutters was omitted. For this demonstration system, the first flush tank is the first treatment component installed for the removal of debris including dust particles, bird droppings, and other foreign objects deposited on the roof during dry days. It is installed to divert the first RW supply containing these contaminants away from the storage tank. Generally, a range between 0.2 and 2.0 mm of the first runoff from the roof must be diverted (Doyle 2008). This value needs to be optimized to receive clean water without losing significant amount of diverted RW. For this demonstration project, 0.25 mm of the first rainwater from the roof was diverted, considering less exposure of the collection area to contaminants, possible loss of large amounts of RW, and application of further treatment methods. With this requirement, two flush tanks with design capacities of 64 L were installed for both 4 m3 tanks. With this installation, 128 L of RW was used for washing the collection roof, which then drained away. A 0.12 m diameter floating ball was used as a floating valve at the top of the flush tank where a 0.16–0.11 m pipe reducer was applied (Figure 3). Finally, a 0.02 m gate valve is installed at the bottom of each first flush tank for drainage purposes.

Sedimentation

Complete particulate removal cannot be achieved by first flush. To further reduce the presence of suspended particles in the supply line, settling is permitted in the tank. Horizontal tanks with sufficient settling area were selected in lieu of vertical tanks of similar capacity to allow for sufficient settling in a relatively short time. The horizontal tanks used for this system had a 3-m-long bed width and a length of 1.5 m.

Calm inlet

To prevent the effects of inlet water turbulence and to reduce the occurrence of particulate re-suspension, laminar flow at the inlet was maintained by installing a calm inlet using a 0.05 m diameter pipe and elbow configuration (Figure 3). This installation dampens the entrance gravity force of the inlet rainwater to limit the extent of re-suspension of settled particles in the tank.

CONCLUSION

Despite its ancient practice, RWH is not yet favored well in Ethiopia. The main challenges of RWH systems in Ethiopia can be solved through improvements and designs following modern engineering approaches. This paper investigated the performance of a RWH system for a demonstration project installed on a three-story building in ASTU. Engineering design procedures are discussed as a means to improve the design gap on existing systems in Ethiopia and further facilitate replicability of the system. NWD, RUE, and CN were analyzed for toilet water applications, which considered a minimum and maximum daily demand of 0.13–3 m3/day. Applying the water balance model for the daily rainfall data of Adama city, it was determined that an 8 m3 tank will have a 60% RUE and can achieve 94 NWDs, which is a 67% reduction in annual NWDs. Water quality was also improved with the use of first flush tanks, particulate settling and calm inlets. Finally, the results of this demonstration project indicate that RWH can be applied as an alternative source of water supply in urban areas similar to Adama. Concerned authorities shall not overlook its contribution in filling the gap between water supply and demand for cities.

ACKNOWLEDGEMENTS

This research was financially supported by the Korea Environmental Industry and Technology Institute (KEITI) and Institute for Global Social Responsibility at Seoul National University (SNU.IGSR).

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